1. Field of the Invention
The present invention relates generally to a coding/decoding apparatus and method in an OFDM (Orthogonal Frequency Division Multiplexing) mobile communication system, and in particular, to a coding/decoding apparatus and method using an STTD (Space-Time block coding based Transmit Diversity) technique.
2. Description of the Related Art
An OFDM technique currently used for high-speed data transmission over a wire/wireless channel, a technique for transmitting data using multiple carriers, is a kind of MCM (Multi-Carrier Modulation) technique, which converts a stream of serial input symbols into parallel symbols and modulates each of the converted parallel symbols with a plurality of orthogonal subcarriers (or subchannels).
A system supporting the MCM technique, called an “MCM system,” was first applied to a high-frequency radio for military use, in the late 1950's, and tests on the OFDM technique for overlapping a plurality of orthogonal subcarriers have been made since the 1970's (see S. B. Weinstein and P. M. Ebert, Data Transmission By Frequency Division Multiplexing Using The Discrete Fourier Transform, IEEE Trans. on Commun., vol. 19, no. 4, pp. 628-675, October 1971). However, due to the difficulty in realizing orthogonal modulation between multiple carriers, the OFDM technique was hardly applied to an actual system. However, after Weinstein et al. proposed in 1971 that OFDM modulation/demodulation could be efficiently performed using DFT (Discrete Fourier Transform), active research has been carried out on the OFDM technique. In addition, as a technique of using a guard interval and inserting a cyclic prefix guard interval becomes generally known, it has become possible to reduce bad influences on the system due to multipath and delay spread interference. Therefore, the OFDM technique has been widely applied to such digital transmission techniques as DAB (Digital Audio Broadcasting), digital television, WLAN (Wireless Local Area Network), WATM (Wireless Asynchronous Transfer Mode), and fixed BWA (Broadband Wireless Access). That is, the OFDM technique was not widely used due to its hardware complicity. However, as various digital signal processing techniques including FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform) have recently been developed, it has become possible to realize the OFDM technique. The OFDM technique, though similar to the conventional FDM (Frequency Division Multiplexing) technique, is characterized in that it has optimal transmission efficiency during high-speed data transmission by maintaining orthogonality between the multiple subcarriers. In addition, the OFDM technique, having high frequency utilization efficiency and strong resistance to multipath fading, is advantageous in that it has optimal transmission efficiency during high-speed data transmission. Further, the OFDM technique, because it overlaps frequency spectrums, has high frequency utilization efficiency and strong resistance to frequency selective fading and multipath fading, and can reduce inter-symbol interference (ISI) by utilizing a guard interval. In addition, it is possible to design an equalizer having a simple structure and strong resistance to impulse noises. Due to the advantages stated above, there is a growing tendency for the OFDM technique to be widely used for the communication systems.
Now, a transmitter and a receiver of a mobile communication system supporting the OFDM technique (hereinafter, referred to as “OFDM mobile communication system”) will be described in brief.
In an OFDM transmitter, input data is modulated with subcarriers through a scrambler, an encoder, and an interleaver. Here, the transmitter provides a variety of variable rates and has a coding rate, an interleaving size and a modulation technique, which can be changed according to a data rate. Commonly, the encoder uses a coding rate of ½ and ¾, and the interleaving size for preventing a burst error is determined according to the number of coded bits per OFDM symbol (NCBPS). The modulation technique includes QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), 16QAM (16-ary Quadrature Amplitude Modulation), and 64QAM (64-ary Quadrature Amplitude Modulation), according to the data rate. A predetermined number of pilots are added to the signal modulated with a predetermined number of subcarriers. The pilot-added signal undergoes IFFT, generating one OFDM symbol. Thereafter, a guard interval for preventing the inter-symbol interference in the multipath channel environment is inserted in the OFDM symbol, and the guard interval-inserted OFDM symbol is finally applied to an RF (Radio Frequency) processor through a symbol wave generator, and then transmitted over a channel.
In an OFDM receiver corresponding to the transmitter, a reverse operation of the operation performed by the transmitter is performed and a synchronization process is added. First, the receiver performs a process of estimating a frequency offset and a symbol offset of a received OFDM symbol by utilizing a training symbol. Thereafter, a guard interval-eliminated data symbol is restored to a predetermined number of pilot-added subcarriers through an FFT block. In addition, in order to overcome a propagation delay phenomenon on an actual wireless channel, an equalizer estimates a channel condition of a received channel signal and eliminates signal distortion on the actual wireless channel from the received channel signal. The channel estimated data through the equalizer is converted to a bit stream, and then output as final data through a deinterleaver, a decoder for error correction, and a descrambler.
Although the OFDM technique has a strong resistance to frequency selective fading, its performance is restricted. A typical improved technique proposed to overcome the restriction of performance is an OFDM mobile communication system using multiple antennas. However, in general, a receiver supporting a radio data service is restricted in its size and power, so it is not preferable for the receiver to include the multiple antennas. For this reason, an improved transmission diversity technique provides a plurality of transmission antennas to the transmitter instead of providing a plurality of reception antennas to the receiver, thus reducing complexity of the receiver and preventing performance degradation because of fading.
Among many transmission techniques developed up to now, the STTD technique has relatively less calculations and low realization complexity. In addition, the OFDM technique is the most suitable communication technique to which the STTD technique is applied, and can rapidly transmit a large amount of data while sacrificing a frequency band the least, and while overcoming multipath interference.
FIG. 1 illustrates a transmitter in a conventional OFDM mobile communication system. The transmitter illustrated in FIG. 1 is designed for an OFDM mobile communication system supporting the STTD technique.
Referring to FIG. 1, the transmitter encodes input data into coded bits at a given coding rate, and interleaves the coded bits, thus generating data 110. The generated data 110 is provided to a modulator (or QPSK/QAM mapper) 120. Although various coding techniques have been proposed, the transmitter typically employs a coding technique using a turbo code, or an error correction code. Further, the transmitter uses a coding rate of ½ and ¾. The modulator 120 modulates the input data 110 by a predetermined modulation technique, and outputs modulated symbols. Here, the modulation technique includes QPSK, 8PSK, 16QAM, and 64QAM, and each of the modulation techniques performs modulation by its unique symbol mapping techniques. It will be assumed in FIG. 1 that QPSK and QAM are used as the modulation technique. The modulated symbols output from the modulator 120 are provided to a space-time block code encoder 130.
The space-time block code encoder 130 encodes the modulated symbols with a space-time block code by mapping the modulated symbols to the space-time block code. An output signal of the space-time block code encoder 130 is provided to two transmission diversity paths. That is, the output signal of the space-time block code encoder 130 is provided to a first IFFT block 140 and a second IFFT block 150. The first and second IFFT blocks 140 and 150 each generate an OFDM symbol by performing IFFT on subcarriers encoded by the space-time block code. The OFDM symbols output from the first and second IFFT blocks 140 and 150 are provided to first and second guard interval inserters 160 and 170, respectively. The first guard interval inserter 160 and the second guard interval inserter 170 insert guard intervals in the OFDM symbols output from the first IFFT block 140 and the second IFFT block 150, respectively. Transmission of the OFDM symbol is commonly performed in a block unit. However, the OFDM symbol is affected by a previous symbol, while it is transmitted over a multipath channel. In order to prevent interference between the OFDM symbols, the guard interval is inserted between consecutive blocks. The guard interval-inserted OFDM symbols from the first and second guard interval inserters 160 and 170 are transmitted over a multipath channel through first and second RF processors 180 and 190, and first and second antennas ANT1 and ANT2.
FIG. 2 illustrates a receiver in a conventional OFDM mobile communication system. The receiver illustrated in FIG. 2 is designed for an OFDM mobile communication system supporting the STTD technique, and has a structure corresponding to the structure of the transmitter illustrated in FIG. 1.
Referring to FIG. 2, a signal transmitted from a transmitter over a multipath channel is received at a first RF processor 210 and a second RF processor 220 through a first antenna ANT1 and a second antenna ANT2, respectively. The first and second RF processors 210 and 220 down-convert the RF signals received through the first and second antennas ANT1 and ANT2 into IF (Intermediate Frequency) signals, and provide the IF signals to first and second guard interval eliminators 230 and 240, respectively. The first guard interval eliminator 230 and the second guard interval eliminator 240 eliminate guard intervals inserted into the OFDM symbols output from the first RF processor 210 and the second RF processor 220, respectively. The guard interval-eliminated OFDM symbols from the first and second guard interval eliminators 230 and 240 are provided to first and second FFT blocks 250 and 260, respectively. The first and second FFT blocks 250 and 260 generate symbols encoded by the space-time block code, through an FFT process. The symbols encoded by the space-time block code are provided to a space-time block code decoder 270, where the provided symbols are decoded by a space-time block code. The modulated symbols decoded by the space-time block code are provided to a demodulator (or QPSK/QAM demapper) 280. The demodulator 280 demodulates the decoded demodulated symbols by a demodulation technique corresponding to the modulation technique used by the transmitter, and outputs coded bits. The coded bits are output as original data 290 through deinterleaving and decoding. Since the modulator 120 in the transmitter uses the modulation techniques of QPSK and QAM, the demodulator 280 also uses demodulation techniques of QPSK and QAM.
In FIGS. 1 and 2, the transmitter and the receiver each use two antennas ANT1 and ANT2 to support the transmission diversity, by way of example. However, it would be obvious to those skilled in the art that the transmitter and the receiver can use more than two antennas.
If the OFDM mobile communication system uses N subcarriers, the signals output from the first and second FFT blocks 250 and 260 in the receiver illustrated in FIG. 2 can be represented byr(k)=H(k)X(k)+n(k), 0≦k≦N−1  Equation (1)
Equation (1) can be rewritten in a determinant, as followsr=H·X+n  Equation (2)
In Equation (2), r denotes an N×1 reception symbol vector, X denotes an N×1 transmission symbol vector, n denotes an N×1 noise vector, and H denotes an N×N diagonal matrix representing a frequency response of a channel.
A description of the FFT blocks 250 and 260 will be separately made herein below for a case where the receiver has one antenna and another case where the receiver has a plurality of antennas, e.g., NR antennas.
(1) One Reception Antenna Used
When the receiver receives, through one antenna, a signal transmitted by a space-time block code for two transmission antennas in the transmitter, a vector of the signal transmitted through the two transmission antennas can be calculated by
                              r          _                =                              [                                                                                r                    1                                                                                                                    r                    2                    *                                                                        ]                    =                                                                      [                                                                                                              H                          1                                                                                                                      H                          2                                                                                                                                                              H                          2                          *                                                                                                                      -                                                      H                            1                            *                                                                                                                                ]                                ⁡                                  [                                                                                                              X                          1                                                                                                                                                              X                          2                                                                                                      ]                                            +                              [                                                                                                    n                        1                                                                                                                                                n                        2                                                                                            ]                                      =                                                            H                  _                                ·                                  X                  _                                            +                              n                _                                                                        Equation        ⁢                                  ⁢                  (          3          )                    
In Equation (3), a superscript “*” represents an operator for complex conjugating each element of the matrix. Further, H1 and H2 represent a frequency response of each channel, and X1 and X2 represent a vector of each transmission symbol. Therefore, a decoded signal is calculated by multiplying the symbol vector by Hermitian of a channel matrix H due to orthogonality of a space-time block code, as follows
                                                                        X                ~                            =                            ⁢                                                                                          H                      _                                        H                                    ⁢                                      r                    _                                                  =                                                                                                    H                        _                                            H                                        ⁢                                          HX                      _                                                        +                                                            H                      H                                        ⁢                                          n                      _                                                                                                                                              =                            ⁢                              [                                                                                                                                                                                                                                                    H                                1                                                            ⁡                                                              (                                0                                )                                                                                                                                          2                                                +                                                                                                                                                                        H                                2                                                            ⁡                                                              (                                0                                )                                                                                                                                          2                                                                                                            0                                                              ⋯                                                                                      ⋯                        ⁢                                                                                                                                                              ⋯                                                              0                                                                                                  0                                                              ⋰                                                              ⋮                                                              ⋮                                                              ⋮                                                              ⋮                                                                                                  ⋮                                                              0                                                                                                                                                                                                                                      H                                1                                                            ⁡                                                              (                                                                  N                                  -                                  1                                                                )                                                                                                                                          2                                                +                                                                                                                                                                        H                                2                                                            ⁡                                                              (                                                                  N                                  -                                  1                                                                )                                                                                                                                          2                                                                                                            0                                                              ⋯                                                              0                                                                                                  0                                                              ⋯                                                              0                                                                                                                                                                                                                                      H                                1                                                            ⁡                                                              (                                0                                )                                                                                                                                          2                                                +                                                                                                                                                                        H                                2                                                            ⁡                                                              (                                0                                )                                                                                                                                          2                                                                                                            ⋯                                                              0                                                                                                  0                                                              ⋯                                                              0                                                              ⋰                                                              ⋰                                                              0                                                                                                  0                                                              ⋯                                                              ⋯                                                              ⋯                                                              0                                                                                                                                                                                                                                      H                                1                                                            ⁡                                                              (                                                                  N                                  -                                  1                                                                )                                                                                                                                          2                                                +                                                                                                                                                                        H                                2                                                            ⁡                                                              (                                                                  N                                  -                                  1                                                                )                                                                                                                                          2                                                                                                                    ]                                                                                                      ⁢                                                                                                                                                              X                          1                                                                                                                                                              X                          2                                                                                                                                      +                                                                            H                      _                                        H                                    ⁢                                      n                    _                                                                                                          Equation        ⁢                                  ⁢                  (          4          )                    
Therefore, the received signal, after being decoded by the space-time block code, becomes equivalent to a signal by which the sum of power of the respective channels is multiplied, thereby obtaining a second-order diversity gain.
(2) NR Reception Antennas Used
When the receiver has a plurality of antennas, signals received through the plurality of antennas are decoded by the space-time block code, and then the decoded signals are summed up. This can be expressed as
                                                                        X                ~                            =                            ⁢                                                ∑                                      m                    =                    1                                                        N                    R                                                  ⁢                                                      [                                                                                                                        H                                                          1                              ⁢                              m                                                                                                                                                            H                                                          2                              ⁢                              m                                                                                                                                                                                                        H                                                          2                              ⁢                              m                                                        *                                                                                                                                -                                                          H                                                              1                                ⁢                                m                                                            *                                                                                                                                            ]                                    ⁡                                      [                                                                                                                        r                                                          1                              ⁢                              m                                                                                                                                                                                                        r                                                          2                              ⁢                              m                                                        *                                                                                                                ]                                                                                                                          =                            ⁢                                                                    ∑                                          m                      =                      1                                                              N                      R                                                        ⁢                                                                                    H                        _                                            m                      H                                        ⁢                                                                  H                        _                                            m                                        ⁢                                          X                      _                                                                      +                                                                            H                      _                                        m                    H                                    ⁢                                                            n                      _                                        m                                                                                                          Equation        ⁢                                  ⁢                  (          5          )                    
In Equation (5), H1m indicates a frequency response of a channel between a first reception antenna and an mth reception antenna, and H2m indicates a frequency response of a channel between a second reception antenna and the mth reception antenna. Therefore, when the receiver has NR reception antennas, the received signal, after being decoded by the space-time block code, obtains a diversity gain of 2NR.
As stated above, the OFDM mobile communication system is designed to overcome the inter-symbol interference caused by the wireless channel. However, the OFDM mobile communication system is not so resistive to signal attenuation due to a multipath phenomenon of the wireless channel. In order to prevent performance deterioration due to the fading channel, an OFDM mobile communication system supporting the STTD technique has been proposed.
In the proposed OFDM mobile communication system, a transmitter uses a plurality of antennas, contributing to a remarkable reduction in complexity of a receiver during system realization. However, the OFDM mobile communication system supporting the STTD technique is restricted in its performance according to the number of transmission antennas. That is, since performance of the OFDM mobile communication system supporting the STTD technique is determined according to the number of transmission antennas, it is necessary to increase the number of transmission antennas in order to increase the system performance. For example, if the number of transmission antennas is increased to 3, the system performance will be remarkably increased as compared with when the number of the transmission antennas is 2. However, in the OFDM mobile communication system supporting the STTD technique, the increase in number of the transmission antennas causes an increase in calculations and a reduction in a data rate. Therefore, in the OFDM mobile communication system supporting the STTD technique, if the number of transmission antennas is increased to 3 or more in order to improve the system performance, the system complexity is increased and the data rate is decreased undesirably.